Abstract/Summary

Early Ordovician acritarch (microphytoplankton) assemblages from an outer shelf to slope facies in northern England show changes in diversity that parallel changes in sea level, with high diversity occurring during lowstands and low diversity during highstands. The pattern is the reverse of that recorded in the literature, in which high acritarch diversity in continental platform successions has been related to periods of transgression and highstand, and low diversity to periods of regression and lowstand. The apparent contradiction can be explained if high-diversity assemblages move further onto continental platforms during transgression and highstand, but towards basins during regression and lowstand.
The hypothesis that high-diversity acritarch assemblages track onshore and offshore during episodes of sea-level change implies that there is a direct causal link between sea level and microphytoplankton diversity, at least in so far as changes in sea level influence the location of high diversity. The direct physical influence of sea-level change on phytoplankton diversity might have been restricted to relatively small scales, however, its most obvious effect being perhaps to shift the locus of maximum diversity rather than affect the overall level of diversity. At larger temporal and spatial scales, the direct influence of sea-level change on acritarch diversity is less certain. Parallel, large-scale changes in sea level and phytoplankton diversity have been documented for the Phanerozoic as a whole, but possibly reflect either independent responses to other factors such as the effects of tectonic cycles and changes in palaeogeography, ocean circulation and climate, or else indicate an indirect influence in the sense that sea-level rise created conditions under which phytoplankton diversity could increase.
In contrast to the results discussed in this paper, other analyses of Ordovician acritarch diversity, at larger spatial scales than those considered here, have concluded that there is no evidence for any causal link between sea-level change and diversity. If, however, the controls on diversity exerted directly by changes in sea level are scale-dependent and only apparent at relatively small scales, analyses at larger spatial scales that compile measures of diversity across a range of depositional environments, i.e. that sample both onshore and offshore environments and combine the results, might not evince a correlation between changes in sea level and diversity.
In addition to producing shifts in the location of high-diversity assemblages, sea-level changes also appear to be accompanied by at least local turnover in species composition. During the Early Ordovician in northern England, the high-diversity assemblages associated with lowstands are very different in composition. Elsewhere, in cratonic settings, maximum flooding intervals within successive Early Palaeozoic stratigraphic sequences are characterised by markedly different acritarch assemblages. Hence, changes in sea level appear to be associated with local extinctions and originations as well as changes in the locus of high diversity, possibly as a result of ecological stresses arising from changes in the location of acritarch assemblages.